One of my readers left a note in the comments asking me for advice on how to build a solar lighting system to run as barn lights in the evening. This article shows how I would go about building one, and includes the math on how to adjust this idea for your own setup.

Before I go into too much detail, here is what the original reader sent me:

Chris, I would like to light my barn for feeding, or working after dark. I would like 2-4′ fluorescent lights. I was thinking of solar panels, batteries, & inverter. What’s your recommendation for a set up like this?

Thanks, Jeff

Well Jeff, here is how I would go about creating solar barn light fixtures. I went ahead and assumed 2 foot lights. If you need more barnlight, just duplicate the 2 foot setup, or rework the math below to compensate for the power (watts) used by a 4 foot bulb. These kinds of indoor solar lighting applications can get expensive quickly. That’s another reason for assuming 2 foot lights: I want want to keep costs down.

Choosing Bulbs for Solar Barnyard Lighting

The first place to start is to pick the bulbs you want to use. Once you pick those, then you can size the battery properly. With the lights and batteries nailed down, you can figure out how big of a solar panel you’ll need.

Doing a quick look around Amazon, I found these 2 foot, 14 watt bulbs that should work nicely as a 12v fluorescent light. Most lighting systems will take either two or four bulbs. Since I can’t know which kind you’ll have, I’ll just do the math per bulb. Just add these power/amperage numbers for each bulb you want to operate.

The first step is to figure out how much current this bulb is going draw from your battery. The light is rated for 14 watts, but for the sake of simpler math, we’ll add 4 watts. This compensates for losses through wiring, the inverter, etc. This will make a pessimistic estimate, which is good. At 18 watts of power, you’ll be drawing 1.5 amps from a 12 volt battery:

Choosing Batteries for Solar Lights

Now, you need to estimate what the longest amount of time is that you’ll want to run the barnlight for. Since you mentioned working after dark, I’ll assume 4 hours is the maximum amount of time you’d run them. A 1.5 amp draw for 4 hours works out to be 6 amp-hours.

Knowing the amp-hour draw, you can properly size your battery. Remember: discharging a battery below 80% of its rating will cause deep-discharge damage. Be sure to multiply a batteries amp-hour rating by 0.8 to compensate for this.

Diving the 6 amp-hours (per bulb for 4 hours of operation) into the 60 amp-hour of effective capacity of the battery, we know that this battery can power 10 bulbs (for 4 hours). It could also power fewer bulbs for longer, but the take away from all this math is that a deep-cycle Group 24 battery should be able to power your barn lights.

Choosing an Inverter For Your Solar Lighting System

It’s also important to pay attention to the power rating of your inverter. Assuming you wanted to take this setup to the max (10 bulbs), and each bulb is consuming 18 watts of power, then you need an inverter that can put out 180 watts of power. You could get away with a 200 watt inverter, but this 400 watt inverter is inexpensive and has gotten some great reviews on Amazon:

Choosing Panels For Your Solar Barn

Whew! So we’ve got the lights, the battery, and the inverter nailed down. You can finally choose a properly sized solar panel. Since the size of the panel will determine how quickly the battery will charge, I have to make some some more assumptions as to how you want to use you solar lighting system. Typically, I would assume that you want to use your lights on a day-to-day basis in the winter. This means the batteries have to be fully charged in a day and that we may only have the equivalent of 6 hours of light on the solar panels. This may still not be enough in the dead of winter, but it’s a good design target.

The number of lights you run will also play a role here. If you power fewer lights than 10, you won’t need as large of a panel. In fact, for this part of the design, I’ll assume you only need enough power from your solar panel to replace the power consumed by 4 bulbs. This means that the panel only needs to recharge the battery over the course of 2 days, which means you can use a significantly cheaper solar panel. A bulb at 18 watts for 4 hours means you’ll consume 72 watt-hours per bulb:

Assuming you run 4 bulbs, you’ll have to replace 288 watt-hours of power each day:

Assuming the sun only shines on the solar panel for 6 hours in the winter, this means you’ll need a 48 watt panel or larger:

You’ll also need a charge controller. You’ll fry your battery if you connect this panel up to it directly. I’m a fan of the Sunforce charge controller, but the Kintrex controller has a low-voltage cut-off that will protect your battery from deep discharge damage. This feature will allow you to get significantly longer life from your battery.

Assembling the Barn Light Fixtures

Once you have all your subcomponents, you’ll need to assemble them like in this diagram. What you’re effectively building is a solar power generator, designed to drive your 12v fluorescent lights.

7 Responses to A DIY Barn Solar Lighting System

We live in western Washington State and we’re having a 36’x36′ barn built at the beginning of next month. I’d like to avoid paying someone to come out later this year to run power to the barn, as well as preventing an increase in my power bill. I’d like to be able to utilize solar for lighting as well as a few outlets for running a tankless water heater occasionally, (for bathing the horses), and a small fridge, (for storing vaccinations). I’d also like to run my electric fence charger. I haven’t been able to find any information regarding the use of outlets in a stand alone system. Any information you have would be great!

An update: Just a few hours after I posted my comment, a co-worker gave me an inverter he wasn’t using. Its a Trace T1112SB, so 1100W, 12V. I found the manual online, and discovered that its the truck model, meaning semi trucks. I don’t know if I’d still be able to use this for my purposes. I hope so, because it was free and in working order.

Yes, you should be able to use that inverter with out much trouble. You should be able to follow the equations on this page and scale the system for your needs. You’ll need to measure the amp draw for any additional utilities (water pump, fridge, etc) and factor those into the load calculations. I would recommend starting small and building up from there. Follow the direction on this page and install a few lights, then add batteries and panels and you can scale up to the rest of your loads.

I had a few more questions regarding this setup. One, could you recommend a light fixture to fit the bulbs you referenced earlier? Two, could I connect two batteries together or would I have to upgrade to a larger one? Three, would I be wiring a light switch to operate the lights? I hope I’m not bothering you too much with my questions. I’ve never wired anything more complicated than slash-wire, and my husband never learned anything electrical. Thanks!

Sorry to take so long to respond. Work has been busy and I’ve been dedicating most of my free time to my boat. I’m excited to hear that you are getting serious about getting into solar! Here are the answers to your questions:

2. Yes, you can connect multiple batteries together in parallel (all +’s connected together) to add capacity to your batteries.

3. The inverter will require some standby power when its turned on. Instead of wiring in a light switch, you could just connect/disconnect the inverter or wire in a switch for it. You don’t want to leave your inverter turned on if you’re not using it.

Hi Chris, I have a question about the above plan for the solar powered barn… If the lights are 12 volts why do you need an inverter? Doesn’t the inverter switch it over to 110 volts? I’m planning on doing the same type system for my barn but I was hoping to use all 12 volt LED fixtures and use a 12 volt marine circuit breaker panel. I’m trying to avoid the loss of power when converting to 110 volts. The only time I would need 110 power would be for using clipper or other power tools and this would be on rare occasions so I was thinking of installing a 12 volt cigarette lighter type adapter somewhere in the system that I could plug a small portable inverter into for my occasional 110 needs. Would this work?

That’s an excellent idea. You are correct in that you don’t need an inverter if you run directly off 12 volts. Using 12v LED lighting is more efficient. And yes, using a small inverter for your power tools would also be a great idea. Just be sure to pay attention to the ‘peak power’ or ‘inrush current’ rating of the inverter as power tools often require an inverter that can withstand frequent power surges.

The original commenter who asked me about designing a barn lighting system wanted to use florescent lights, so that’s why I mentioned the use of an inverter. I’m not aware of 12 volt ballasts for florescent lights.